Fine-Tuning of the Optical and Electrochemical Properties of Ruthenium(II) Complexes with 2-Arylbenzimidazoles and 4,4′-Dimethoxycarbonyl-2,2′-bipyridine

A series of cyclometalated complexes of ruthenium (II) with four different substituents in the aryl fragment of benzimidazole was synthesized in order to study the effect of substituent donation on the electronic structure of the substances. The resulting complexes were studied using X-ray diffraction, NMR spectroscopy, MALDI mass spectrometry, electron absorption spectroscopy, luminescence spectroscopy, and cyclic voltammetry as well as DFT/TDDFT was also used to interpret the results. All the complexes have intense absorption in the range of up to 700 nm, the triplet nature of the excited state was confirmed by measurement of luminescence decay. With an increase in substituent donation, a red shift of the absorption and emission bands occurs, and the lifetime of the excited state and the redox potential of the complex decrease. The combination of these properties shows that the complexes are excellent dyes and can be used as photosensitizers.


Introduction
Ruthenium (II) polypyridine complexes have a wide range of applications: they are used as photocatalysts [1], luminescent chemosensors [2], dyes in Grätzel cells [3][4][5], anticancer drugs [6], etc.The main feature of these complexes is the presence of intense absorption in the visible region, which makes them very promising for usage in light conversion molecular devices.Classic complexes for this purpose are isothiocyanate complexes with monodentate NCS ligands [7].However, such architecture has poor stability, due to which they are inappropriate for use in solutions.As an alternative bidentate and tridentate N-donor ligands based on pyridine and pyrazole rings are used instead of isothiocyanate ligands [8,9].Another way to increase the stability of complexes is cyclometalation, i.e., the formation between the metal and carbon atoms from the ligand.Here and below, cyclometalated ligands will be referred to as CˆN ligands, since they are coordinated to the ruthenium atom by carbon and nitrogen, and N-donor bidentate ligands as NˆN ligands.It is important to note that when a cyclometalated fragment is introduced into the complex, photophysical and electrochemical properties of the latter dramatically change [8].Therefore, the analysis of factors affecting the properties of complexes is an urgent task.One approach to the search for new dyes is to vary the donor-acceptor nature of substituents in CˆN ligands.The highest occupied molecular orbital (HOMO) is localized on the d-orbitals of ruthenium and the orbitals of the CˆN ligand [8,10,11].The lowest unoccupied molecular orbital (LUMO) is localized mainly on the NˆN ligand.Therefore, the change in the electronic structure of the ligand is directly related to the HOMO-LUMO gap, and, as a result, to the position of the absorption edge, the emission maximum, and the redox potential.The most studied cyclometalated complexes are the ones bearing phenylpyridines, phenylazoles, or terpyridines [7,[12][13][14][15][16][17][18][19][20][21][22][23][24][25].
Nevertheless, phenylpyridines are complex objects for synthesis, and there are no papers where strong electron-donor substituents have been introduced into them, although the effect of electron-withdrawing substituents has been extensively studied [14][15][16].As an alternative class of ligands, benzimidazoles can be used.These compounds are convenient objects for precise changes in the electronic structure since they are readily available synthetically, and it is possible to introduce a wide range of substituents at different positions of the aryl and benzimidazole fragments.
Molecules 2023, 28, 6541 2 of 20 conversion molecular devices.Classic complexes for this purpose are isothiocyanate complexes with monodentate NCS ligands [7].However, such architecture has poor stability, due to which they are inappropriate for use in solutions.As an alternative bidentate and tridentate N-donor ligands based on pyridine and pyrazole rings are used instead of isothiocyanate ligands [8,9].Another way to increase the stability of complexes is cyclometalation, i.e., the formation between the metal and carbon atoms from the ligand.Here and below, cyclometalated ligands will be referred to as C^N ligands, since they are coordinated to the ruthenium atom by carbon and nitrogen, and N-donor bidentate ligands as N^N ligands.It is important to note that when a cyclometalated fragment is introduced into the complex, photophysical and electrochemical properties of the latter dramatically change [8].Therefore, the analysis of factors affecting the properties of complexes is an urgent task.One approach to the search for new dyes is to vary the donor-acceptor nature of substituents in C^N ligands.The highest occupied molecular orbital (HOMO) is localized on the d-orbitals of ruthenium and the orbitals of the С^N ligand [8,10,11].The lowest unoccupied molecular orbital (LUMO) is localized mainly on the N^N ligand.Therefore, the change in the electronic structure of the ligand is directly related to the HOMO-LUMO gap, and, as a result, to the position of the absorption edge, the emission maximum, and the redox potential.The most studied cyclometalated complexes are the ones bearing phenylpyridines, phenylazoles, or terpyridines [7,[12][13][14][15][16][17][18][19][20][21][22][23][24][25].Nevertheless, phenylpyridines are complex objects for synthesis, and there are no papers where strong electron-donor substituents have been introduced into them, although the effect of electron-withdrawing substituents has been extensively studied [14][15][16].As an alternative class of ligands, benzimidazoles can be used.These compounds are convenient objects for precise changes in the electronic structure since they are readily available synthetically, and it is possible to introduce a wide range of substituents at different positions of the aryl and benzimidazole fragments.
Complexes with ligands of the second type (Figure 1b) have also been studied as sensitizers for DSSC [38].For example, the authors of [38] studied the effect of the size of the conjugated system of CˆN-ligand on the photosensitizer efficiency.
Finally, there are several studies where complexes with ligands of the fourth type (Figure 1d) have been investigated [48][49][50].For example, in [49], ruthenium complexes with 2-arylbenzimidazoles modified with a carboxyl group were investigated as anticancer drugs.In [50], cyclometalated ruthenium complexes with unsubstituted 2-phenylimidazole and various NˆN-donor ligands were proposed as photosensitizers.The closest to the topic of this work is the study of cyclometalated ruthenium complexes with 1-benzyl-2aryl-benzimidazoles, which showed good performance in DSSC, where -CF 3 groups and N-hexylphenothiazine [48], i.e., only acceptor substituents, were introduced into the aryl moiety.The introduction of the strong acceptor N-hexylphenothiazine shifts the absorption bands to the red region, but the cell sensitized by it shows lower efficiency relative to the sensitizer with -CF 3 groups.Cyclometalated iron complexes have also been studied with similar ligands as sensitizers for DSSCs [51].In addition to the ligands used in [48], the authors introduced a donor-NMe 2 group into the aryl fragment, which led to the desired destabilization of the MC-state.
In addition to ruthenium(II) complexes, benzimidazoles have been used as CˆN ligands in iridium(III) complexes [52].It is shown that the expansion of the conjugated ligand system and its geometry have a significant effect not only on the electronic structure of the complex, but also on its structure and composition [53][54][55].
Previously, we studied cyclometalated complexes of ruthenium(II) [56] and iridium(III) [57] with 1-phenyl-2-aryl-benzimidazoles and showed that the benzimidazole and aryl parts of the ligand strongly differ from each other in donor-acceptor properties, which leads to the prevalence of ligand-to-ligand charge transfer (LLCT) in molecules.As a way to solve this problem, we propose to introduce an electron-donor substituent (e.g., methyl) into the benzimidazole fragment.Methyl radical was chosen because it is the simplest sigma electron-donating group.Then, 4,4 -dimethoxycarbonyl-2,2 -bipyridine (dmdcbp) was chosen as the NˆN ligand.Usually, 4,4 -dicarboxy-2,2 -bipyridine (dcbp) is used as an anchoring ligand.We use it not as an acid but as an ester, as this increases the solubility of the complexes and facilitates their study.In our previous work [56], it was shown that the hydrolysis is easily carried out in one step and that the optical properties of the complex with dcbp and dmdcbp do not differ practically.
In this work, we have designed and synthesized a series of cyclometalated Ru(II) complexes with 2-aryl-5-methylbenzimidazoles bearing electron-withdrawing (NO 2 ) and electron-donating substituents (OMe, NMe 2 ) and anchoring 4,4 -dimethoxycarbonyl-2,2bipyridine.We studied the composition and structure of complexes with NMR spectroscopy, MALDI mass spectrometry, and X-ray diffraction.Photophysical properties were studied with UV-vis and luminescence spectroscopy, redox potentials were measured by use of cyclic voltammetry (CV).These results were interpreted together with their electronic structure, obtained by a combined DFT/TD-DFT approach.The results were compared to our previous work [56].

Synthesis and Characterization
CˆN ligands were prepared via 3-step synthesis from 4-methyl-2-nitroaniline (Scheme 1).The ligands were obtained with a high yield by condensation of N-benzyl-4-methyl-ophenylenediamine and bisulfite adducts of corresponding aldehydes with various substituents using standard procedure [58].The desired diamine was produced by the reduction of 4-methyl-2-nitro-N-benzylaniline with hydrazine using Raney nickel [59].The precursor for reduction was obtained by benzylation of 4-methyl-2-nitroaniline with benzylchloride (BnCl) [60].This route of synthesis was chosen in order to avoid isomer formation.
by the reduction of 4-methyl-2-nitro-N-benzylaniline with hydrazine using Raney nickel [59].The precursor for reduction was obtained by benzylation of 4-methyl-2-nitroaniline with benzylchloride (BnCl) [60].This route of synthesis was chosen in order to avoid isomer formation.Complexes 1-4 were prepared via a standard [56] 2-stage method (Scheme 2).In the first stage, cyclometalation was carried out.The product is unstable in solution; therefore, it was used in the second stage without characterization, and the composition of the complex was assigned in accordance with the literature [25].The solid contained some amount of p-cymene, which was not evaporated under reduced pressure.That is why the shortage of dmdcbp was taken on the 2nd stage-it is hard to separate the final product from the unreacted dmdcbp.It is interesting to note that an increase in the acceptor properties of a substituent leads to a decrease in the yield of cyclometalation.In the second stage, the dimethyl ester of dicarboxybipyridine (dmdcbp) was introduced into the complex.We decided to use ester instead of acid in order to simplify the isolation and purification of complexes.The resulting compounds were characterized by 1 H NMR, HRMS, complexes 1 and 3 were studied by X-ray crystallography.

NMR Spectroscopy
The composition and purity of the complexes were verified by 1 H NMR spectroscopy.The spectrum of the free C^N ligand is very different from that of the cyclometalated one.First, one of the protons of the aryl fragment disappears as a result of the formation of a ruthenium-carbon covalent bond.This also causes the signals of neighboring protons to change.Second, the formation of a covalent bond with a metal leads to a redistribution of the electron density in the ligand and, as a consequence, to a strong change in the chemical shift of the ligand proton signals.The proton signals of the free N^N ligand also differ greatly from the coordinated one (Figure 2).It is interesting to note that bipyridine rings become non-equivalent when coordinated with a metal.This is clearly seen in signals in the region of 8.9-9.1 ppm.Protons from four different bipyridyl rings have different chemical shifts due to different Ru-N bond lengths.One of the signals is shifted more than the others because its proton is located in the pyridyl ring, the nitrogen of which is farthest from the ruthenium (Table 1).Complexes 1-4 were prepared via a standard [56] 2-stage method (Scheme 2).In the first stage, cyclometalation was carried out.The product is unstable in solution; therefore, it was used in the second stage without characterization, and the composition of the complex was assigned in accordance with the literature [25].The solid contained some amount of p-cymene, which was not evaporated under reduced pressure.That is why the shortage of dmdcbp was taken on the 2nd stage-it is hard to separate the final product from the unreacted dmdcbp.It is interesting to note that an increase in the acceptor properties of a substituent leads to a decrease in the yield of cyclometalation.In the second stage, the dimethyl ester of dicarboxybipyridine (dmdcbp) was introduced into the complex.We decided to use ester instead of acid in order to simplify the isolation and purification of complexes.The resulting compounds were characterized by 1 H NMR, HRMS, complexes 1 and 3 were studied by X-ray crystallography.
by the reduction of 4-methyl-2-nitro-N-benzylaniline with hydrazine using Raney nickel [59].The precursor for reduction was obtained by benzylation of 4-methyl-2-nitroaniline with benzylchloride (BnCl) [60].This route of synthesis was chosen in order to avoid isomer formation.Complexes 1-4 were prepared via a standard [56] 2-stage method (Scheme 2).In the first stage, cyclometalation was carried out.The product is unstable in solution; therefore, it was used in the second stage without characterization, and the composition of the complex was assigned in accordance with the literature [25].The solid contained some amount of p-cymene, which was not evaporated under reduced pressure.That is why the shortage of dmdcbp was taken on the 2nd stage-it is hard to separate the final product from the unreacted dmdcbp.It is interesting to note that an increase in the acceptor properties of a substituent leads to a decrease in the yield of cyclometalation.In the second stage, the dimethyl ester of dicarboxybipyridine (dmdcbp) was introduced into the complex.We decided to use ester instead of acid in order to simplify the isolation and purification of complexes.The resulting compounds were characterized by 1 H NMR, HRMS, complexes 1 and 3 were studied by X-ray crystallography.

NMR Spectroscopy
The composition and purity of the complexes were verified by 1 H NMR spectroscopy.The spectrum of the free C^N ligand is very different from that of the cyclometalated one.First, one of the protons of the aryl fragment disappears as a result of the formation of a ruthenium-carbon covalent bond.This also causes the signals of neighboring protons to change.Second, the formation of a covalent bond with a metal leads to a redistribution of the electron density in the ligand and, as a consequence, to a strong change in the chemical shift of the ligand proton signals.The proton signals of the free N^N ligand also differ greatly from the coordinated one (Figure 2).It is interesting to note that bipyridine rings become non-equivalent when coordinated with a metal.This is clearly seen in signals in the region of 8.9-9.1 ppm.Protons from four different bipyridyl rings have different chemical shifts due to different Ru-N bond lengths.One of the signals is shifted more than the others because its proton is located in the pyridyl ring, the nitrogen of which is farthest from the ruthenium (Table 1).Scheme 2. Synthesis of the complexes 1-4.

NMR Spectroscopy
The composition and purity of the complexes were verified by 1 H NMR spectroscopy.The spectrum of the free CˆN ligand is very different from that of the cyclometalated one.First, one of the protons of the aryl fragment disappears as a result of the formation of a ruthenium-carbon covalent bond.This also causes the signals of neighboring protons to change.Second, the formation of a covalent bond with a metal leads to a redistribution of the electron density in the ligand and, as a consequence, to a strong change in the chemical shift of the ligand proton signals.The proton signals of the free NˆN ligand also differ greatly from the coordinated one (Figure 2).It is interesting to note that bipyridine rings become non-equivalent when coordinated with a metal.This is clearly seen in signals in the region of 8.9-9.1 ppm.Protons from four different bipyridyl rings have different chemical shifts due to different Ru-N bond lengths.One of the signals is shifted more than the others because its proton is located in the pyridyl ring, the nitrogen of which is farthest from the ruthenium (Table 1).Single crystals of compound 1 were obtained from CH2Cl2:CHCl3 1:1 mixture under slow solvent evaporation; single crystals of complex 3 suitable for X-ray diffraction study were obtained from CH2Cl2:hexane 3:1 mixture.
The asymmetric unit of crystal structure of 1 contains C^N cyclometalated [Ru(L-NO2)(dmdcbp)2] + complex cation, PF6 − anion and 5 solvated chloroform molecules, while the asymmetric unit of crystal structure of 3 consists of C^N cyclometalated [Ru(L-(OMe)2)(dmdcbp)2] + complex cation, PF6 − anion, and solvated hexane molecule (Figure 3).In both structures, the ruthenium atom is located in a slightly distorted octahedral coordination environment and is coordinated by two nitrogen atoms of each of the bipyridine ligands and one nitrogen and one carbon atom of the C^N ligand; the list of selective bonds is presented in Table 1.

Crystal Structures
Single crystals of compound 1 were obtained from CH 2 Cl 2 :CHCl 3 1:1 mixture under slow solvent evaporation; single crystals of complex 3 suitable for X-ray diffraction study were obtained from CH 2 Cl 2 :hexane 3:1 mixture.
The asymmetric unit of crystal structure of 1 contains CˆN cyclometalated [Ru(L-NO 2 )(dmdcbp) 2 ] + complex cation, PF 6 − anion and 5 solvated chloroform molecules, while the asymmetric unit of crystal structure of 3 consists of CˆN cyclometalated [Ru(L-(OMe) 2 )(dmdcbp) 2 ] + complex cation, PF 6 − anion, and solvated hexane molecule (Figure 3).In both structures, the ruthenium atom is located in a slightly distorted octahedral coordination environment and is coordinated by two nitrogen atoms of each of the bipyridine ligands and one nitrogen and one carbon atom of the CˆN ligand; the list of selective bonds is presented in Table 1.
It is to be noted that the Ru1-N6 bond trans to Ru1-C1 is elongated compared to other Ru-N bonds, which can be explained by the trans effect [61].According to the analysis of the distribution of Ru-C bond lengths in CˆN cyclometalated complexes published in the Cambridge Structure Database (CSD version 5.44, update June 2023), Ru1-C1 bond length is typical.Analysis of the crystal packing of 1 (Figure 4) is additionally stabilized by weak intermolecular interactions between the carbon atom of solvated chloroform and the carboxyl oxygen atom of the dimethyl ester of dicarboxybipyridine (C1S-H1S. ..O3, C1S. ..O3 distance is 3.14 Å).In the meantime, the crystal packing of 3 did not reveal any notable intermolecular interactions, but revealed that the disordered hexane molecule is located within channels situated along the c axis (Figure 5).According to the powder XRD data, due to the loss of solvate molecules, the crystals decay during storage.It is to be noted that the Ru1-N6 bond trans to Ru1-C1 is elongated compared to other Ru-N bonds, which can be explained by the trans effect [61].According to the analysis of the distribution of Ru-C bond lengths in C^N cyclometalated complexes published in the Cambridge Structure Database (CSD version 5.44, update June 2023), Ru1-C1 bond length is typical.Analysis of the crystal packing of 1 (Figure 4) is additionally stabilized by weak intermolecular interactions between the carbon atom of solvated chloroform and the carboxyl oxygen atom of the dimethyl ester of dicarboxybipyridine (C1S-H1S…O3, C1S…O3 distance is 3.14 Å).In the meantime, the crystal packing of 3 did not reveal any notable intermolecular interactions, but revealed that the disordered hexane molecule is located within channels situated along the c axis (Figure 5).According to the powder XRD data, due to the loss of solvate molecules, the crystals decay during storage.It is to be noted that the Ru1-N6 bond trans to Ru1-C1 is elongated compared to other Ru-N bonds, which can be explained by the trans effect [61].According to the analysis of the distribution of Ru-C bond lengths in C^N cyclometalated complexes published in the Cambridge Structure Database (CSD version 5.44, update June 2023), Ru1-C1 bond length is typical.Analysis of the crystal packing of 1 (Figure 4) is additionally stabilized by weak intermolecular interactions between the carbon atom of solvated chloroform and the carboxyl oxygen atom of the dimethyl ester of dicarboxybipyridine (C1S-H1S…O3, C1S…O3 distance is 3.14 Å).In the meantime, the crystal packing of 3 did not reveal any notable intermolecular interactions, but revealed that the disordered hexane molecule is located within channels situated along the c axis (Figure 5).According to the powder XRD data, due to the loss of solvate molecules, the crystals decay during storage.

Optical Properties
UV-vis spectra of complexes were measured in acetonitrile solutions at room temperature (Figure S6).There are strong absorption bands at the UV region (π→π* electronic transitions) at 400-450 nm and 550-650 nm (MLCT transitions), and less intense absorption in the 650-750 nm range (see Figure 6).The nature of the latter will be discussed in the quantum chemical calculations section.The absorption spectra are decomposed into Gaussian components (Figure S7) to determine the energy of the S 0 →S 1 transition (λ abs 1 ) (see Tables 2 and S1).For complex 3 in the decomposition into Gaussian components, there is no low-energy maximum similar to the other three complexes.However, in the calculated spectrum there is a transition to 797 nm with a very low oscillator strength.Perhaps its intensity is so low that it is difficult to observe it in the absorption spectrum.For the rest of the complexes, the lowest energy maximum approximately coincides in energy with the theory.

Optical Properties
UV-vis spectra of complexes were measured in acetonitrile solutions at room temperature (Figure S6).There are strong absorption bands at the UV region (π→π* electronic transitions) at 400-450 nm and 550-650 nm (MLCT transitions), and less intense absorption in the 650-750 nm range (see Figure 6).The nature of the latter will be discussed in the quantum chemical calculations section.The absorption spectra are decomposed into Gaussian components (Figure S7) to determine the energy of the S0→S1 transition (λabs 1 ) (see Tables 2 and S1).For complex 3 in the decomposition into Gaussian components, there is no low-energy maximum similar to the other three complexes.However, in the calculated spectrum there is a transition to 797 nm with a very low oscillator strength.Perhaps its intensity is so low that it is difficult to observe it in the absorption spectrum.For the rest of the complexes, the lowest energy maximum approximately coincides in energy with the theory.The absorption bands with the maxima at λabs 1 and λabs 2 of the complexes shift to longer wavelengths as the donor properties of the ligand increase.This can be explained by the fact that the HOMO of the complex is localized on the benzimidazole ligand; therefore, the introduction of donor substituents increases the HOMO energy, while the introduction of acceptor substituents lowers it.
All the complexes exhibit luminescence in the near-IR region (800-950 nm).The spectra measured at 298 K for compounds 3 and 4 with donor substituents have lowintensity bands at a higher energy region of 600-800 nm.However, these bands are The absorption bands with the maxima at λ abs 1 and λ abs 2 of the complexes shift to longer wavelengths as the donor properties of the ligand increase.This can be explained by the fact that the HOMO of the complex is localized on the benzimidazole ligand; therefore, the introduction of donor substituents increases the HOMO energy, while the introduction of acceptor substituents lowers it.
All the complexes exhibit luminescence in the near-IR region (800-950 nm).The spectra measured at 298 K for compounds 3 and 4 with donor substituents have low-intensity bands at a higher energy region of 600-800 nm.However, these bands are negligible in comparison with the emission bands located at 800-950 nm in the spectra recorded at 77 K.It can be assumed that regarded bands correspond to emission from the singlet state.The lifetime of this state could not be measured because of the low intensity of the band.At room temperature, the emission maxima of complexes 1-3 peaked at a wavelength of about 900 nm and slightly blue-shifted in the spectra recorded at 77 K (see Figure 7).Substituents in ligands affect the energy of the emitted excited state with a consequence red shift of the emission band by 81 nm (0.14 eV) (Table 2) from complex 1 to complex 4. In conclusion, the emission intensity of the long-wavelength band greatly increases upon cooling in comparison with the intensity of the band at 600-800 nm (Figure S3), which indicates the triplet nature of the excited state.
To prove the proposed hypothesis, the luminescence decays at room temperature and 77 K with registration at emission maxima were measured.Decay curves for complexes 1-3 are well-fitted by a monoexponential function (see Figures S4 and S5), whereas luminescence decay of compound 4 reveals bi-exponential behavior.For the compounds with bi-exponential relaxation of the excited state, the short-time component was employed for further analysis by virtue of significantly higher amplitude (see Figure S4).Excited state lifetimes were estimated as several nanoseconds for the compounds at 298 K.However, at 77 K the lifetimes greatly increase, which also testifies in favor of the assumption about the triplet nature of the excited state.The 392 ± 2 ns lifetime for complex 1 is an unusually long time for fluorescence and instead can be assigned to phosphorescence from T 1 state.The vibrational relaxation of the T 1 state energy is fully suppressed at 77 K leading to observable lifetime increase.It should be noted that the lifetime decreases from complex 1 (392 ± 2 ns) to complex 4 (88 ± 1 ns) for both the decays recorded at 77 and 298 K.It is associated with an increase in the donation of substituents.
Molecules 2023, 28, 6541 9 of 20 phosphorescence from T1 state.The vibrational relaxation of the T1 state energy is fully suppressed at 77 K leading to observable lifetime increase.It should be noted that the lifetime decreases from complex 1 (392 ± 2 ns) to complex 4 (88 ± 1 ns) for both the decays recorded at 77 and 298 K.It is associated with an increase in the donation of substituents.   1 The maximum of the first intense absorption band. 2 The maximum of the low-energy band.* Measured at λ abs 2 .

Quantum Chemical Calculations
To further investigate the influence of the R substituents in the ligand on the electronic properties of the corresponding complexes, quantum chemical calculations were performed for the compounds in the gas phase at the M06 levels of theory with the def2-SV(P) basis set for all atoms except ruthenium.Large-core energy-adjusted quasi-relativistic RECP for Ru, developed by the Stuttgart and Dresden groups, along with the accompanying basis set ECP28MWB, was used.The optimized ground-state geometries for the compounds are in good agreement with the X-ray crystal structures (Table S1).Cartesian coordinates are given in Table S2.It should be noted that the phenyl fragment in the Bn unit is twisted by approximately 22 • .This difference can be attributed to crystal packing effects.The variation of the substituent units does not significantly change the geometry of the whole complexes, as demonstrated by X-ray single crystal analysis.
The calculated frontier molecular orbital LUMO is predominantly located on the NˆN ligand (92%) for all the compounds (see Figure 8).In contrast, the HOMO is presumed to be located on Ru for complexes 1 (67%) and 2 (60%), while for complexes 3 and 4, the HOMO is located on CˆN (61% and 80%, respectively).We observed a trend of decreasing Ru(II) ion contribution in the HOMO along with an increase in CˆN ligand contribution from compound 1 to 4 (see Figure 8).The redistribution of molecular orbital localization from the Ru(II) ion to the CˆN ligand, specifically on the motif with the R group, leads to an increase in HOMO energy from −8.0 to −7.4 eV.
To gain a deeper insight into electronic excitation processes, TD-DFT calculations were performed at the same level of theory.The calculated S 0 →S 1 energies agree well with the experimentally estimated values, with an energy deviation not exceeding 0.03 eV (in the case of complex 2).According to the calculations, the S 0 →S 1 electronic transition for complexes 1 and 2 has the nature of a metal-to-ligand charge transfer (MLCT) state, involving a transition from Ru to NˆN with an additional contribution from a transition from CˆN to NˆN ligand.Thus, we assume an MLCT (Ru→NˆN) + ligand-to-ligand charge transfer (LLCT) (CˆN→NˆN) nature for this absorption band.However, compounds 3 and 4, which contain electron-donating substituents, demonstrate an LLCT (CˆN→NˆN) nature for the S 0 →S 1 transition.The initial orbital is located on the NMe 2 -containing unit of the CˆN ligand.
The first intense absorption band, located in the region of 500-600 nm (see Figure 6), has the nature of an MLCT from the d orbital of Ru to the π* orbital of NˆN (Ru→NˆN), as determined by calculations.The calculated absorption energies of 577-606 nm do not exceed 0.1 eV compared to those obtained from the deconvolution of experimental spectra.The UV-Vis spectra simulated based on the TD results are quite similar to the measured spectra.
The estimated energies of the first excited triplet state T 1 are close to the experimental values (Table 3), slightly lower by only 0.05-0.11eV.Since the S 0 →T 1 transition is predominantly from HOMO to LUMO, the T 1 state is an MLCT (Ru→NˆN) state.Therefore, it is supported that the introduction of an electron-withdrawing NO 2 unit increases the T 1 energy, while the introduction of electron-donating motifs decreases it.
spectra.The UV-Vis spectra simulated based on the TD results are quite similar to the measured spectra.
The estimated energies of the first excited triplet state T1 are close to the experimental values (Table 3), slightly lower by only 0.05-0.11eV.Since the S0→T1 transition is predominantly from HOMO to LUMO, the T1 state is an MLCT (Ru→N^N) state.Therefore, it is supported that the introduction of an electron-withdrawing NO2 unit increases the T1 energy, while the introduction of electron-donating motifs decreases it.

Electrochemical Studies
CVs obtained for the complexes in a wide range of potentials have the same form (Figure 9).Reversible redox peak at high potentials changes depending on the electrondonating effect of CˆN-ligand-the potential lowers with an increase in electron-donating properties.This correlates with the change in HOMO level.The redox peaks at low potentials range practically do not change their position between complexes (Figure S9).This behavior we associate with the electrochemical properties of NˆN ligands.

Materials and Methods
All commercially available reagents were at least reagent grade and used without further purification.Solvents were distilled and dried according to standard procedures.
NMR spectra were acquired at 25 °C on a Bruker Avance 600 spectrometer and chemical shifts were reported in ppm referenced residual solvent signals.
Single-crystal X-ray diffraction analysis of 3 and 1 was carried out on a Bruker D8 Quest (Bruker, Billerica, MA, USA) diffractometer (MoKα radiation, ω and φ-scan mode).The structure was solved with direct methods and refined by the least squares method in the full-matrix anisotropic approximation on F 2 .All hydrogen atoms were located in calculated positions and refined within the riding model.All calculations were performed using the SHELXTL (version NT) and Olex2 [63][64][65]    We also estimated the energies of HOMO and LUMO using the oxidation and reduction potentials (Table 4).To do this, we added to the obtained potentials the absolute potential of the pair Fc/Fc + = 5.1 eV [62].The energy gap (Eg) was calculated as the difference between oxidation and reduction potentials.The dependence of the HOMO and LUMO energies determined in this way on the substituent in the ligand is the same as in the case of quantum chemical calculations.E LUMO for all complexes is approximately the same, except for complex 1, as in its case, it is lower.This can be explained by the fact that the introduction of a strong acceptor into one ligand leads to a decrease in the electron density on the NˆN ligand as well.That is why the E gap is the same for complexes 1 and 2, although complex 2 does not contain acceptor substituents.

Materials and Methods
All commercially available reagents were at least reagent grade and used without further purification.Solvents were distilled and dried according to standard procedures.
NMR spectra were acquired at 25 • C on a Bruker Avance 600 spectrometer and chemical shifts were reported in ppm referenced residual solvent signals.
Single-crystal X-ray diffraction analysis of 3 and 1 was carried out on a Bruker D8 Quest (Bruker, Billerica, MA, USA) diffractometer (MoKα radiation, ω and ϕ-scan mode).The structure was solved with direct methods and refined by the least squares method in the full-matrix anisotropic approximation on F 2 .All hydrogen atoms were located in calculated positions and refined within the riding model.All calculations were performed using the SHELXTL (version NT) and Olex2 [63][64][65] software packages.Atomic coordinates, bond lengths, angles, and thermal parameters have been deposited at the Cambridge 4-methyl-2-nitroaniline (3.77 g, 27.32 mmol), benzylchloride (5 mL, 43.45 mmol, 1.6 eq.), KBr (6 g), and water (55 mL) were added to a 250 mL round-bottomed flask equipped with a reverse refrigerator and an anchor of a magnetic stirrer; the reaction mixture was stirred for 1.5 h at boiling, and then the solution was cooled to room temperature, treated with a saturated solution of sodium bicarbonate (2.45 g), extracted with ethyl acetate (2 × 65 mL), and washed with water (50 mL).The organic phase was evaporated on a rotary evaporator and crystallized from ethanol (Scheme 1).The yield was: 4.73 g (72%) in the form of red-orange crystals. 1

N-benzyl-4-methylphenylendiamine
The synthesis was carried out according to the method described in [59].4-methyl-2-nitro-N-benzylaniline (1.21 g, 5 mmol), obtained Raney nickel, and methanol (24 mL) were added to a round-bottomed flask with a volume of 100 mL, equipped with a reverse refrigerator and an anchor of a magnetic stirrer.Then, hydrazine hydrate (1.25 g, 5 mmol) was added drop by drop.The reaction mixture was stirred for 3-5 h at 70 • C. Excess hydrazine was removed by adding another portion of nickel Raney.The resulting solution was purified from nickel by column chromatography and evaporated on a rotary evaporator (Scheme 1).The yield was 0.8 g, (76%).The target compound was obtained in the form of yellow oil.The obtained substances were used in the next step without characterization because they oxidize rapidly.
General method of obtaining bisulfite adducts Bisulfite adducts were obtained by the method [72].
The corresponding aldehyde was dissolved in a minimum amount of ethanol.A saturated aqueous solution of sodium pyrosulfite was added to the solution with stirring.The resulting suspension was stirred for several minutes, the precipitate was filtered out and washed 2-3 times with cold alcohol.The white powder was dried in a desiccator over phosphorus oxide (V).The outputs are practically quantitative.

General method of synthesis of 2-aryl-N-benzyl-5-methylbenzimidazoles
The synthesis was carried out according to the method in [58].4-methyl-N-benzylphenylenediamine (1 g, 4.75 mmol) dissolved in ethanol (5 mL) and a bisulfite adduct of the corresponding aldehyde (5.7 mmol, 1.2 eq.) dissolved in ethanol (10 mL) were added to a 50 mL round-bottomed flask equipped with a reverse refrigerator and an anchor of a magnetic stirrer.The reaction mixture was stirred for 3-5 h during boiling.At the end of this time, the reaction mixture was cooled and filtered at reduced pressure, and the inorganic phase was removed from the precipitate by washing it with water.Next, the target product was washed off the filter with hot ethanol, the product that fell out of the saturated solution was filtered, and the remainder was crystallized from an aqueous alcohol solution (Scheme 1).NMR spectra are given in Figures S24-S27.

Figure 1 .
Figure 1.Types of coordination of substituted imidazoles to the ruthenium atom described in the literature: (a) N^C^N pincer bisimidazoles; (b) C^N pyridyl-substituted imidazoles; (c) N^N pyridyl-substituted imidazoles; (d) C^N phenyl-substituted imidazoles.

Figure 2 .
Figure 2. Comparison of aromatic region of 1 H NMR spectra of free dmdcbp (up) and coordinated and bound into complex 3 (bottom).

Figure 2 .
Figure 2. Comparison of aromatic region of 1 H NMR spectra of free dmdcbp (up) and coordinated and bound into complex 3 (bottom).

Figure 3 .
Figure 3. General view of 1 (on the left) and 3 (on the right).Atoms are presented as thermal ellipsoids at 50% probability, hydrogen atoms and solvated chloroform (1) and hexane (3) molecules are omitted for clarity.Ruthenium atoms and their coordination environment are labeled.

Figure 4 .
Figure 4. View of the C-H…O bond in 1 (presented as dotted line).Atoms are presented as thermal ellipsoids at 50% probability, hydrogen atoms, and solvated chloroform molecules not involved in intermolecular interactions are omitted for clarity.

Figure 3 .
Figure 3. General view of 1 (on the left) and 3 (on the right).Atoms are presented as thermal ellipsoids at 50% probability, hydrogen atoms and solvated chloroform (1) and hexane (3) molecules are omitted for clarity.Ruthenium atoms and their coordination environment are labeled.

Figure 3 .
Figure 3. General view of 1 (on the left) and 3 (on the right).Atoms are presented as thermal ellipsoids at 50% probability, hydrogen atoms and solvated chloroform (1) and hexane (3) molecules are omitted for clarity.Ruthenium atoms and their coordination environment are labeled.

Figure 4 .
Figure 4. View of the C-H…O bond in 1 (presented as dotted line).Atoms are presented as thermal ellipsoids at 50% probability, hydrogen atoms, and solvated chloroform molecules not involved in intermolecular interactions are omitted for clarity.

Figure 4 .
Figure 4. View of the C-H. ..O bond in 1 (presented as dotted line).Atoms are presented as thermal ellipsoids at 50% probability, hydrogen atoms, and solvated chloroform molecules not involved in intermolecular interactions are omitted for clarity.

Molecules 2023, 28 , 6541 7 of 20 Figure 5 .
Figure 5. Crystal packing of 3 along c axis.Atomic displacement thermal ellipsoids are presented only for C atoms of solvated hexane molecules; hydrogen atoms are omitted for clarity.

Figure 5 .
Figure 5. Crystal packing of 3 along c axis.Atomic displacement thermal ellipsoids are presented only for C atoms of solvated hexane molecules; hydrogen atoms are omitted for clarity.Molecules 2023, 28, 6541 8 of 20

Figure 8 .
Figure 8.The energetic diagram of the complexes and molecular orbitals for compound 1 calculated by DFT approach.Figure 8.The energetic diagram of the complexes and molecular orbitals for compound 1 calculated by DFT approach.

Figure 8 .
Figure 8.The energetic diagram of the complexes and molecular orbitals for compound 1 calculated by DFT approach.Figure 8.The energetic diagram of the complexes and molecular orbitals for compound 1 calculated by DFT approach.

Figure 9 .
Figure 9.Typical CV of 5 mM complex (on example of complex 1) solution in 0.1M TBAP/CH3CN in wide range of potentials.Reversible peak at high potentials is related to Ru 2+ oxidation; multiple peaks at low potentials are associated with acceptor ligand.Sweep rate 100 mV•s −1.

Figure 9 .
Figure 9.Typical CV of 5 mM complex (on example of complex 1) solution in 0.1M TBAP/CH 3 CN in wide range of potentials.Reversible peak at high potentials is related to Ru 2+ oxidation; multiple peaks at low potentials are associated with acceptor ligand.Sweep rate 100 mV•s −1 .

Table 1 .
Bond lengths of ruthenium coordination environment in complexes 1 and 3.

Table 1 .
Bond lengths of ruthenium coordination environment in complexes 1

Table 3 .
The calculated and experimental energies of the absorption wavelengths S 1 with oscillator strength f and the energies of the first excited triplet state T 1 for the investigated compounds.
software packages.Atomic coordinates, bond lengths, angles, and thermal parameters have been deposited at the Cambridge Crystallographic Data Centre with deposition numbers CCDC 2255408 (accesed on 11 April 2023) and 2259495 (accessed on 27 April 2023) , which are available free of charge at www.ccdc.cam.ac.uk.

Table 4 .
Measured electrochemical characteristics for complexes 1